Maskless lithography systems and methods utilizing spatial light modulator arrays

ABSTRACT

A maskless lithography system that writes patterns on an object. The system can include an illumination system, the object, spatial light modulators (SLMs), and a controller. The SLMs can pattern light from the illumination system before the object receives the light. The SLMs can include a leading set and a trailing set of the SLMs. The SLMs in the leading and trailing sets change based on a scanning direction of the object. The controller can transmit control signals to the SLMs based on at least one of light pulse period information, physical layout information about the SLMs, and scanning speed of the object. The system can also correct for dose non-uniformity using various methods.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/449,908, filed May 30, 2003, which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to lithography. Moreparticularly, the present invention relates to maskless lithography.

2. Related Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays (e.g., liquid crystal displays), circuit boards,various integrated circuits, and the like. A frequently used substratefor such applications is a semiconductor wafer or glass substrate. Whilethis description is written in terms of a semiconductor wafer forillustrative purposes, one skilled in the art would recognize that thisdescription also applies to other types of substrates known to thoseskilled in the art.

During lithography, a wafer, which is disposed on a wafer stage, isexposed to an image projected onto the surface of the wafer by exposureoptics located within a lithography apparatus. While exposure optics areused in the case of photolithography, a different type of exposureapparatus can be used depending on the particular application. Forexample, x-ray, ion, electron, or photon lithography each can require adifferent exposure apparatus, as is known to those skilled in the art.The particular example of photolithography is discussed here forillustrative purposes only.

The projected image produces changes in the characteristics of a layer,for example photoresist, deposited on the surface of the wafer. Thesechanges correspond to the features projected onto the wafer duringexposure. Subsequent to exposure, the layer can be etched to produce apatterned layer. The pattern corresponds to those features projectedonto the wafer during exposure. This patterned layer is then used toremove or further process exposed portions of underlying structurallayers within the wafer, such as conductive, semiconductive, orinsulative layers. This process is then repeated, together with othersteps, until the desired features have been formed on the surface, or invarious layers, of the wafer.

Step-and-scan technology works in conjunction with a projection opticssystem that has a narrow imaging slot. Rather than expose the entirewafer at one time, individual fields are scanned onto the wafer one at atime. This is accomplished by moving the wafer and reticlesimultaneously such that the imaging slot is moved across the fieldduring the scan. The wafer stage must then be asynchronously steppedbetween field exposures to allow multiple copies of the reticle patternto be exposed over the wafer surface. In this manner, the quality of theimage projected onto the wafer is maximized.

Conventional lithographic systems and methods form images on asemiconductor wafer. The system typically has a lithographic chamberthat is designed to contain an apparatus that performs the process ofimage formation on the semiconductor wafer. The chamber can be designedto have different gas mixtures and grades of vacuum depending on thewavelength of light being used. A reticle is positioned inside thechamber. A beam of light is passed from an illumination source (locatedoutside the system) through an optical system, an image outline on thereticle, and a second optical system before interacting with asemiconductor wafer.

A plurality of reticles are required to fabricate a device on thesubstrate. These reticles are becoming increasingly costly and timeconsuming to manufacture due to the feature sizes and the exactingtolerances required for small feature sizes. Also, a reticle can only beused for a certain period of time before being worn out. Further costsare routinely incurred if a reticle is not within a certain tolerance orwhen the reticle is damaged. Thus, the manufacture of wafers usingreticles is becoming increasingly, and possibly prohibitively expensive.

In order to overcome these drawbacks, maskless (e.g., direct write,digital, etc.) lithography systems have been developed. The masklesssystem replaces a reticle with a spatial light modulator (SLM) (e.g., adigital micromirror device (DMD), a liquid crystal display (LCD), or thelike). The SLM includes an array of active areas (e.g., mirrors ortransmissive areas) that are either ON or OFF to form a desired pattern.A predetermined and previously stored algorithm based on a desiredexposure pattern is used to turn ON and OFF the active areas.

Conventional SLM-based writing systems (e.g., Micronic's Sigma 7000series tools) use one SLM as the pattern generator. To achieve linewidthand line placement specifications, gray scaling is used. For analogSLMs, gray scaling is achieved by controlling mirror tilt angle (e.g.,Micronic SLM) or polarization angle (e.g., LCD). For digital SLMs (e.g.,TI DMD), gray scaling is achieved by numerous passes or pulses, wherefor each pass or pulse the pixel can be switched either ON or OFFdepending on the level of gray desired. Because of the total area on thesubstrate to be printed, the spacing between active areas, the timing oflight pulses, and the movement of the substrate, several passes of thesubstrate are required to expose all desired areas. This results in lowthroughput (number of pixels packed into an individual opticalfield/number of repeat passes required over the substrate) and increasedtime to fabricate devices. Furthermore, using only one SLM requires morepulses of light or more exposure time to increase gray scale. This canlead to unacceptably low levels of throughput.

Therefore, what is needed is a maskless lithography system and methodthat can expose all desired areas on a substrate for each pattern duringonly one pass of a substrate.

SUMMARY OF THE INVENTION

The present invention provides a maskless lithography system. The systemcan include an illumination system, an object, spatial light modulators(SLMs), and a controller. The SLMs can pattern light from theillumination system before the object receives the light. The SLMs caninclude a leading set and a trailing set of the SLMs. The SLMs in theleading and trailing sets change based on a scanning direction of theobject. The controller can generate control signals to the SLMs based onat least one of light pulse period information, physical layoutinformation about the SLMs, and scanning speed of the object.

Other embodiments of the present invention provide a method forcontrolling dose in maskless lithography. The method includes measuringa dose delivered in each pulse in a series of pulses from SLMs,calculating a dose error based on the measuring steps, calculating acorrectional blanket dose based on the dose error, and applying thecorrectional blanket dose using a final set of SLMs.

Still other embodiments of the present invention include a method forcontrolling dose in maskless lithography. The method includes measuringan intensity of a dose from a leading set of SLMs, subtracting themeasured intensity from a predetermined value to generate an errorsignal, delaying the error signal, adding the delayed signal anotherpredetermined value to generate a control signal, and using the controlsignal to control dose from a trailing set of SLMs.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 shows a maskless lithography system having reflective spatiallight modulators according to embodiments of the present invention

FIG. 2 shows a maskless lithography system having transmission spatiallight modulators according to embodiments of the present invention.

FIG. 3 shows a spatial light modulator according to an embodiment of thepresent invention.

FIG. 4 shows more details of the spatial light modulator in FIG. 3.

FIGS. 5, 6, 7, 8, 9, and 10 show two-dimensional arrays of spatial lightmodulators according to various embodiments of the present invention.

FIG. 11 shows an exposure diagram for sequential pulses of light from anillumination source according to various embodiments of the presentinvention.

FIG. 12 is a system 1200 that can control dose and/or uniformity for amultiple SLM pattern generation array, according to an embodiment of thepresent invention.

FIG. 13 is a flow chart depicting a method according to embodiments ofthe present invention.

FIGS. 14 and 15 show two-dimensional arrays of spatial light modulatorsaccording to various embodiments of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers mayindicate identical or functionally similar elements. Additionally, theleft-most digit(s) of a reference number may identify the drawing inwhich the reference number first appears.

DETAILED DESCRIPTION OF THE INVENTION

Overview

While specific configurations and arrangements are discussed, it shouldbe understood that this is done for illustrative purposes only. A personskilled in the pertinent art will recognize that other configurationsand arrangements can be used without departing from the spirit and scopeof the present invention. It will be apparent to a person skilled in thepertinent art that this invention can also be employed in a variety ofother applications.

An embodiment of the present invention utilizes an array of SLMs in amaskless lithography system in order to allow for multiple exposures tothe same area on an object surface during each scanning pass. Using thearray of SLMs can increase throughput and lower costs compared toconventional maskless systems using only one SLM.

By integrating multiple SLMs into one mechanical assembly, a fieldreplaceable unit can be made. This unit could integrate mechanical andthermal stability, cooling channels, purge gas channels, and electricalconnections. Drive electronics, including wiring, memory, andprocessors, could also be integrated into assembly 500, either on abackside or in the empty space on a front side of assembly 500.

Maskless Lithography Systems

FIG. 1 shows a maskless lithography system 100 according to anembodiment of the present invention. System 100 includes an illuminationsystem 102 that transmits light to a reflective spatial light modulator104 (e.g., a digital micromirror device (DMD), a reflective liquidcrystal display (LCD), or the like) via a beam splitter 106 and SLMoptics 108. SLM 104 is used to pattern the light in place of a reticlein traditional lithography systems. Patterned light reflected from SLM104 is passed through beam splitter 106 and projection optics 110 andwritten on an object 112 (e.g., a substrate, a semiconductor wafer, aglass substrate for a flat panel display, or the like).

It is to be appreciated that illumination optics can be housed withinillumination system 102, as is known in the relevant art. It is also tobe appreciated that SLM optics 108 and projection optics 110 can includeany combination of optical elements required to direct light ontodesired areas of SLM 104 and/or object 112, as is known in the relevantart.

In alternative embodiments, either one or both of illumination system102 and SLM 104 can be coupled to or have integral controllers 114 and116, respectively. Controller 114 can be used to adjust illuminationsource 102 based on feedback from system 100 or to perform calibration.Controller 116 can also be used for adjustment and/or calibration.Alternatively, controller 116 can be used for turning ON and OFF activedevices (e.g., pixels, mirrors, locations, etc.) 302 (see FIG. 3) on SLM104, as was described above, to generate a pattern used to expose object112. Controller 116 can either have integral storage or be coupled to astorage element (not shown) with predetermined information and/oralgorithms used to generate the pattern or patterns.

FIG. 2 shows a maskless lithography system 200 according to a furtherembodiment of the present invention. System 200 includes an illuminationsource 202 that transmits light through a SLM 204 (e.g., a transmissiveLCD, or the like) to pattern the light. The patterned light istransmitted through projection optics 210 to write the pattern on asurface of an object 212. In this embodiment, SLM 204 is a transmissiveSLM, such as a liquid crystal display, or the like. Similar to above,either one or both of illumination source 202 and SLM 204 can be coupledto or integral with controllers 214 and 216, respectively. Controllers214 and 216 can perform similar functions as controller 114 and 116described above, and as known in the art.

Example SLMs that can be used in systems 100 or 200 are manufactured byMicronic Laser Systems AB of Sweden and Fraunhofer Institute forCircuits and Systems of Germany.

Merely for convenience, reference will be made only to system 100 below.However, all concepts discussed below can also apply to system 200, aswould be known to someone skilled in the relevant arts.

FIG. 3 shows details of an active area 300 of SLM 104. Active area 300includes an array of active devices 302 (represented by dotted patternsin the figure). Active devices 302 can be mirrors on a DMD or locationson a LCD. It is to be appreciated that active devices 302 can also bereferred to as pixels, as is known in the relevant art. By adjusting thephysical characteristics of active devices 302, they can be seen asbeing either ON or OFF. Digital or analog input signals based on adesired pattern are used to turn ON and OFF various active devices 302.In some embodiments, an actual pattern being written to object 112 canbe detected and a determination can be made whether the pattern isoutside an acceptable tolerance. If so, controller 116 can be used togenerate analog or digital control signals in real time to fine-tune(e.g., calibrate, adjust, etc.) the pattern being generated by SLM 104.

FIG. 4 shows further details of SLM 104. SLM 104 can include an inactivepackaging 400 surrounding active area 300. Also, in alternativeembodiments, a main controller 402 can be coupled to each SLM controller116 to monitor and control an array of SLMs (see discussion below). Asdiscussed below, adjacent SLMs may be offset or staggered with respectto each other in other embodiments.

Spatial Light Modulator Array Configurations

FIG. 5 shows an assembly 500 including a support device 502 thatreceives an array of SLMs 104. In various embodiments, as described inmore detail below, the array of SLMs 104 can have varying numbers ofcolumns, rows, SLMs per column, SLMs per row, etc., based on a number ofdesired exposures per pulse, or other criteria of a user. The SLMs 104can be coupled to a support device 502. Support device 502 can havethermal control areas 504 (e.g., water or air channels, etc.), areas forcontrol logic and related circuitry (e.g., see FIG. 4 showing elements116 and element 402, which can be ASICs, A/D converters, D/A converters,fiber optics for streaming data, etc.), and windows 506 (formed withinthe dashed shapes) that receive SLMs 104, as is known in the relevantart. Support device 502, SLMs 104, and all peripheral cooling or controldevices are referred to as an assembly. Assembly 500 can allow for adesired step size to produce the desired stitching (e.g., connecting ofadjacent elements of features on object 112) and overlap for leading andtrailing SLMs 104. By way of example, support device 502 can be 250mm×250 mm (12 in×12 in) or 300 mm×300 mm (10 in×10 in). Support device502 can be used for thermal management based on being manufactured froma temperature stable material.

Support device 502 can be utilized as a mechanical backbone to ensurespacing control of SLMs 104 and for embedding the circuitry and thethermal controls areas 504. Any electronics can be mounted on either orboth of a backside and front side of support device 502. For example,when using analog based SLMs or electronics, wires can be coupled fromcontrol or coupling systems 504 to active areas 300. Based on beingmounted on support device 502, these wires can be relatively shorter,which reduces attenuation of analog signals compared to a case where thecircuitry is remote from the support device 502. Also, having shortlinks between the circuitry and active areas 300 can increasecommunication speed, and thus increase pattern readjustment speed inreal time.

In some embodiments, when SLM 104 or electrical devices in the circuitrywear out, assembly 500 can easily be replaced. Although it would appearreplacing assembly 500 is more costly than just a chip on assembly 500,it is in fact easier and quicker to replace the entire assembly 500,which can save production costs. Also, assembly 500 can be refurbished,allowing for a reduction in replacement parts if end users are willingto use refurbished assemblies 500. Once assembly 500 is replaced, onlyverification of the an overall alignment is needed before resumingfabrication. In some examples, kinematic mounting techniques can be usedto allow for repeatable mechanical alignments of assembly 500 duringfield replacements. This may eliminate a need for any optical adjustmentof assembly 500.

FIGS. 6, 7, 8, 9, and 10 show how one exposure area of object 112 ispatterned by a section of an SLM array. Thus, the figures show how thesection of the SLM array will look from the perspective of the oneexposure area of object 112.

FIGS. 6 and 7 show alternative embodiments for how sections 650 and 750of an array of SLMs 104 will fall within exposure areas 660 and 760,respectively. Sections 650 and 750 both include four columns having twoequivalent SLMs 104 each. Thus, sections 650 and 750 include eightequivalent SLMs 104. SLMs 104 in one column can be staggered withrespect to SLMs 104 in adjacent columns. Each column is spaced a widthof one-half an active area 300 apart.

In one example, active area 300 can be 4.8 mm×30 mm and each activedevice 302 can be about 6 μm×6 μm. This can produce about 150×magnification. In this example, if an entire SLM 104 is about 4megapixels (e.g., 4096 active devices 302×1024 active devices 302), eachsection 650 or 750 can be about 797 μm×240 μm and each exposure area 660or 760 can be about 120 mm×36 mm.

In this example, there is about a 4.8 nm step size between light pulsesat a SLM plane and about a 34 μm step between exposure periods at anobject plane. Object 112 can be moving at approximately 128 mm/sec in adirection of arrow A. A data refresh rate and/or pulse rate ofillumination source can be around 4 kHz. With these parameters, anexpected throughput of up to about 5 wafers per hour (wph) can bepossible. Thus, if an object's speed was about one active area widthtraveled per light pulse, each exposure area 660 and 760 would receivetwo pulses of light during each scan period of object 112.

FIG. 8 shows another embodiment of an array of SLMs 104 having a section850 writing to exposure an area 860. Section 850 includes eight columnshaving four SLMs 104 each. Thus, section 850 includes 32 SLMs 104. SLMs104 in one column can be staggered with respect to SLMs 104 in adjacentcolumns. Each column is spaced a width of one-half an active area 300apart.

In one example, active area 300 can be about 8.192 mm×32.768 mm and eachactive device 302 can be about 6 μm×6 μm. This can produce about 400×magnification. In this example, if an entire SLM 104 is about 1megapixel (e.g., 2048 active devices 302×512 active devices 302). eachsection 850 can be about 567.5 μm×344 μm and each exposure area 860 canbe about 227 mm×137.2 mm.

In this example, there is about a 16.4 mm step size between light pulsesat a SLM plane and about a 43.52 μm step between exposure periods at anobject plane. Object 112 can be moving at approximately 40.96 mm/sec ina direction of arrow B. A data refresh rate and/or pulse rate ofillumination source can be around 1 kHz. With these parameters, anexpected throughput of up to about 1.2 wph can be possible. Thus, if anobject's speed was about two active area widths traveled per lightpulse, each exposure area 860 would receive two pulses of light duringeach scan period of object 112. In an alternative example, if anobject's speed was about one active area width traveled per light pulse,each exposure area 860 would receive four pulses of light during eachscan period of object 112.

FIG. 9 shows another embodiment of an array of SLMs 104 having a section950 writing to an exposure area 960. Section 950 includes six columnsalternating between having three or four SLMs 104. Thus, section 950includes 14 SLMs 104. SLMs 104 in one column can be staggered withrespect to SLMs 104 in adjacent columns. Each column is spaced one widthof one active area apart.

In one example, active area 300 can be about 8.192 mm×32.768 mm and eachactive device 302 can be about 6 μm×6 μm. This can produce about 400×magnification. In this example, if an entire SLM 104 is about 1megapixel (e.g., 2048 active devices 302×512 active devices), eachsection 950 can be about 567.5 μm×344 μm and each exposure area 960 canbe about 227 mm×137.2 mm.

In this example, there is about a 8.2 mm step size between light pulsesat a SLM plane and about a 21.76 μm step between exposure periods at anobject plane. Object 112 can be moving at approximately 1 Khz or 20.48mm/sec in a direction of arrow C. With these parameters, an expectedthroughput of up to about 0.6 wph can be possible. Thus, if an object'sspeed was about one active area width traveled per light pulse, eachexposure area 960 can receive two pulses of light during each can periodof object 112.

FIG. 10 shows another embodiment of an array of SLMs 104 having asection 1050 writing to an exposure area 1060. Section 1050 can includetwo columns having four SLMs 104 each. Thus, section 1050 includes 8SLMs 104. SLMs 104 in one column can be staggered with respect to SLMs104 in adjacent columns. Each column is spaced one-half an active areawidth apart.

In one example, active area 300 can be about 4.5 mm×36 mm and eachactive device 302 can be about 6 μm×6 μm. This can produce about 150×magnification. In this example, if an entire SLM 104 is about 4megapixels (e.g., 6000 active devices 302×750 active devices 302), eachsection 1050 can be about 1593 μm×96 μm and each exposure area 1060 canbe about 239 mm×14 mm.

In this example, there is about a 4.5 mm step size between light pulsesat a SLM plane and about a 31.5 μm step between exposure periods at anobject plane. Object 112 can be moving at approximately 64 mm/sec in adirection of arrow D. A data refresh rate and/or pulse rate ofillumination source can be around 4 kHz. With these parameters, anexpected throughput of up to about 5.1 wph can be possible. Thus, if anobject's speed was about one-half active area width traveled per lightpulse, each exposure area 1060 could receive two pulses or light duringeach scan period of an object 112.

Exposure Diagrams for Arrays of Spatial Light Modulators

FIG. 11 is one example of an exposure diagram for three sections 1150 ofan array having four SLMs 104 per section as they write to a same row ofexposure areas 1160 on object 112 during five pulses of light. Sections1150-1 and 1150-3 can be part of a first (e.g., leading) set of SLMs andSection 1150-2 can be part of a second (e.g., trailing) set of SLMs.This exposure diagram is shown from the perspective of object 112 as itis moving in the direction of the arrow with an equivalent step of twowidths of active areas 300 per light pulse. During Pulse 1, the arrayhas not overlapped object 112. During Pulse 2, a pattern generated bythe array for SLMs 104 in a first section 1150-1 is written to a firstexposure area 1160-1. During Pulse 3, either the same or a differentpattern is written to exposure area 1160-1 by section 1150-2 and eitherthe same or different pattern is written to exposure area 1160-2 bysection 1150-1. Thus, the trailing set in section 1150-2 writes over asame exposure area 1160-1 later in time as the leading set in section1150-1. This general exposure process is repeated for Pulses 4 and 5, asis shown.

It is to be appreciated this is a very simple example of the exposureprocess that can occur using an array of SLMs 104 in a masklesslithography system. It is being used to demonstrate how using an arrayof SLMs 104 allows for multiple exposures in each exposure area 1160during each scan period, which increases throughput compared to aconventional system using one SLM.

Operation

In this example, light is scanned across object 112, while each SLM 104receives updated pattern data. This results in multiple pulsesreflecting from multiple SLMs 104 as scanning occurs. In each direction,a first set (e.g., a leading set) of SLMs 104 directs a first pulse andsecond set (e.g., a trailing set) of SLMs 104 comes up behind the firstset and directs the second pulse (e.g., trailing SLMs). Hence, at anyinstance in time a single pulse is directed by varying pattern profileson SLMs 104 to write varying patterns to object 112.

For example, during the duration between pulses, object 112 is steppedeither all or a portion of a width of active area 300. Then, 3-4 pulseslater, a trailing SLM 104 can overlap something printed 3-4 pulses agoby a leading SLM 104. System 100 can continuously or periodically updatethe pattern, accordingly. This allows for printing during multiplepasses with SLMs 104, while keeping object 112 continuously moving andonly doing one pass over object 112 to achieve higher throughputcompared to conventional systems using only one SLM.

In essence, system 100 allows for exposing multiple patterns during onepass by using multiple SLMs 104. There could be full overlap, halfoverlap, etc. of patterns generated by leading and trailing SLMs 104 inorder to allow for stitching or other effects.

Some features of the various embodiments of the present inventiondescribed above may be that it allows for: process flexibility in termsof number of pulses to deliver each dose, while maintaining acontinuously moving wafer, easy algorithm development for patternrasterization by pre-defining the geometric relationship between leadingand trailing SLMs, dead pixels on one SLM to be compensated for bycorresponding pixels on other SLMs, and a mechanism by which an array ofmultiple SLMs can be field-replaceable on a single mechanical unit withonly minor electrical, mechanical, pneumatic, and cooling connectionsand a quick optical adjustment.

The geometrical layout of assembly 500 (e.g., the spaces between SLMs104) can be a function of: active area 300 on each SLM 104, the areataken up by packaging 400 for each SLM 104, the number of pulses desiredto deliver a particular dose to a particular exposure area, the maximumobject stage speeds achievable, and the maximum lens diameter inprojection optics 110.

In one example, an amount of exposures for each exposure area can beincreased by a factor of two (i.e. 2, 4, 8, 16, etc.) using the same SLMarray layouts by just halving the object stage scan speed. Scan speedshould remain constant, and is defined by the geometrical relationshipbetween SLMs 104. The amount of overlap between leading and trailingSLMs 104 depends on the overall stitching strategy employed. Differentexamples of this include full overlap, half overlap, or shifted overlap(e.g., full or half overlap where the pixels on trailing SLMs 104 areoffset by a fraction of a pixel in X and Y as compared to leading SLMs104). The spacing between leading and trailing SLMs can be on the orderof the smallest possible multiple of(active_area_width)/(#_of_exposures), plus the stitching overlap,compatible with the physical packaging of the SLM.

Dose and Uniformity Control System and Method Using Monitoring

FIG. 12 is a system 1200 that can control dose and/or uniformity for amultiple SLM pattern generation array, according to an embodiment of thepresent invention. The control of SLMs 104 can be based uponmeasurements 1202 of an intensity using controller 1204 (e.g., adose/uniformity manipulator for leading pattern generation SLMs (hightransmission measured)). Controller 1204 measures leading SLMs 104 atthe point in time that the leading SLMs 104 are exposed. Thismeasurement is subtracted from a predetermined value 1206 (e.g.,setpoint/dose uniformity value in leading SLMs) using subtractor 1208 togenerate an error signal 1210 (e.g., dose/uniformity error in leadingSLMs 104. Error signal 1210 can be delayed using delay device 1212 thatreceives a delay signal 1214. Delay signal 1214 can be based on thenumber of pulses between leading and trailing pulses of the SLM array.The delayed signal 1212 is added to a predetermined value 1216 (e.g., asetpoint dose/uniformity value in the trailing SLMS) using adder 1218 togenerate a control signal 1220. Controller 1222 receives control signal1220, which can be a dose/uniformity manipulator for the trailingpattern generation SLMs 104. Controller 1222 may be low transmissioncontrollable.

If the controlled SLM 104 has sufficient zones, it can also be used tovary intensity along the height of the exposure for the trailing SLM 104to compensate for non-uniformities in the beam during the leading pulse.To accommodate stitching, which may cause two “first pulses”to beoverlapped with one “second pulse,” the trailing portion can be furthersubdivided into bands that are commanded with the appropriatecorrection. The shot energy in the trailing SLMs 104 can be selected soas to accomplish stitching.

In order to successfully compensate for dose variations during theleading pulse without worrying about induced errors from trailingpulses, the energy in the leading pulse can be significantly higher thanthe trailing pulses. As an example for a two-pulse system, a ratio of90% dose for leading SLMs 104, 10% dose for trailing SLMs 104 could beenvisioned, meaning that the error in dose on the trailing SLMs 104would be 9× lower than the error in dose on the leading SLM 104.Continuing the example, if the dose on a given set of leading SLMs 104was measured at 85% instead of the 90% nominal, the attenuation of thetrailing SLMs 104 during the appropriate pulse could be set to allow 15%dose transmission, instead of the nominal 10% dose.

The SLM 104 can be constructed to cover both sides of the beam. Thiswould allow for reversal in exposure scan direction (which reverses theleading and trailing SLMs 104) as well as to provide the capability forcorrecting offsets in transmission and uniformity for the leading SLMs104.

It is to be appreciated that this concept is readily extendable to asingle SLM system or any functional multi-SLM array, and can be used inany lithographic printing strategy with two or more pulses per point onthe wafer being applied to deliver dose. One advantage for thisembodiments is that it can improve dose control in a direct-writelithographic system use of conventional lithographic lasers, which haverelatively poor pulse-to-pulse energy intensity variability anduniformity performance.

Dose Control System and Method Using a Correctional Blanket Dose

In maskless lithography only a very limited number of laser pulses areused to expose the resist. This is to maintain a reasonable throughputin a maskless lithography tool. For example, a number of laser flashesexposing the resist can be limited to 2 to 4 at each site on the wafer.The dose repeatability of the commonly used excimer laser is typicallyin the 1 to 3% 1σ, while the required exposure dose needs to be within0.5% 3σ. Without monitoring this would result in an unacceptable dosevariations.

Embodiments of the present invention can use 3 or 4 laser flashes(exposures) in which the last pulse only contains a small (e.g., 5%)fraction of the total dose needed to expose the resist. Although anexample system and method are found in WO 99/45435, embodiments of thepresent invention can have several advantages over this system, such assubstantially no throughput loss and very limited increase in the costof goods manufactured.

Embodiments of the present invention divide the dose over the laserflashes, such that the last flash only delivers a small fraction, say5%, of the total dose. Measuring the first two or three doses thendefines the dose in the last pulse.

In one example, the last exposure can have the full patterninginformation. In this case, the data path needs to be fully loaded togenerate that information. Moreover, if the exposures are deliveredsequentially, as has been done in conventional systems, the lastexposure decreases the throughput of the tool considerably. In anotherexample, the last exposure can have substantially no patterninginformation, as is described below.

Accordingly, embodiments on the present invention provide a finalexposure to correct for dose errors in the previous exposures. The finalexposure will be delivered as a blanket exposure. This means that thefinal exposure does not contain any pattern information. The finalexposure thus does not need an extensive (and thus expensive) data path.

FIG. 13 is a flow chart depicting a method 1300 according to embodimentsof the present invention. In step 1302, a first set of SLMs measures adose delivered in the first pulse. In step 1304, a second set of SLMsmeasures a dose delivered in a second pulse. In step 1306, a dose erroris calculated. In step 1308, a correctional blanket dose is calculated.In step 1310, the correctional blanket dose is applied through a finalset of SLMs.

FIG. 14 shows a layout of SLMs having 21 SLMs. The SLMs generate threeshots 1402, 1404, and 1406 at three different exposure times. In thisconfiguration, a lens (not shown) in a projection optical system canhave a diameter of about 271 mm.

FIG. 15 shows a layout of SLMs having 24 SLMs. The SLMs generate threeshots 1502, 1504, and 1506 at three different exposure times. In thisconfiguration, a lens (not shown) in a projection optical system canhave a diameter of about 302 mm.

In the proposed layout for FIGS. 14 and 15, all three exposures(1402-1406 or 1502-1506) are done within a single exposure field. Ateach position on the wafer the dose is delivered sequentially. Forexample, in FIG. 15 if pulse N delivers the first dose then pulse N+3,N+4, N+5, or N+6 delivers the second dose, depending on the preciselayout of the SLM array. This means that a single SLM-sized field willexperience four potentially different doses. The SLM in the 3rd shot(column) 1406/1506 should then be delivering four different correctionaldoses. Exposures 1402/1502 and 1404/1504 both hold complete datainformation. This means that SLM columns 1402/1502 and 1404/1504 areconnected to an extensive data path.

The data path is one of the most expensive components of the masklesslithography tool. In conventional systems, an addition of the final shotwould increase the costs even more because it would add about 50% to thedata path. To avoid these extra costs, embodiments of the presentinvention apply a blanket exposure with the final SLM column 1406/1506.This means that the final exposure will contain no pattern data. Thepurpose is to add an additional background in a controlled manner. Theonly “pattern” on the SLMs is there because it will need to correct forpotentially four different doses within the SLM field. This, however isa very simple pattern that needs only a very limited amount ofelectronics.

Consider an aerial image f(x) and a resist threshold th. The boundaryx_(th) between exposed and non-exposed resist is then given by:f(x _(th))=th.  (1)

Now assume that the delivered dose deviates from the ideal dose by afactor b, i.e.:delivered_dose(x)=bf(x).  (2)

Clearly (1) does not hold anymore. To restore the condition laid down in(1), add (1−b) th to (2) and obtain:dose(x)=bf(x)+(1−b)th=th+b(f(x)−th).  (3)

Now dose(x_(th))=th producesth+b(f(x _(th))−th)=th,  (4)

which implies (1). Therefore the correctional background dose is givenby:D=(1−b)th,  (5)

which is independent of the actual pattern. This holds for every valueof b. In our case however, b will be close to but less then one. As anexample the dose in the first two exposures can be 96% and in the finalnominally 4%. Then b will be 0.96. The dose correction method asproposed above does have a small negative effect on the exposurelatitude. The exposure latitude is given by the slope of the aerialimage at the resist threshold: $\begin{matrix}{S = {\frac{\mathbb{d}y}{\mathbb{d}x}❘_{x = x_{th}}}} & (6)\end{matrix}$

Again assume the dose deviates from the ideal dose by a factor b then Swill be given by: $\begin{matrix}{S = {{b\frac{\mathbb{d}y}{\mathbb{d}x}}❘_{x = x_{th}}}} & (7)\end{matrix}$

So the exposure latitude will be degraded by a factor 1−b. In theexample given above the degradation will be 4% (e.g. from 10% to 9.6%).Error budgeting indicates that this decrease of exposure latitude can beabsorbed. However, in preferred embodiments the correctional dose ismaintained as small as possible.

Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A maskless lithography system, comprising: an illumination system; anobject; spatial light modulators (SLMs) that pattern light from theillumination system before the light is received by the object, the SLMsincluding a leading set and a trailing set of the SLMs, the SLMs in theleading and trailing sets changing based on a scanning direction of theobject; and a controller that transmits control signals to the SLMsbased on at least one of light pulse period information, physical layoutinformation about the SLMs, and scanning speed of the object.
 2. Thesystem of claim 1, wherein the object is a semiconductor wafer.
 3. Thesystem of claim 1, wherein the object is a glass substrate.
 4. Thesystem of claim 1, wherein the glass substrate is part of a liquidcrystal display.
 5. The system of claim 1, further comprising: a beamsplitter that directs light from the light source to the SLMs and fromthe SLMs to the object, wherein the SLMs are reflective SLMs.
 6. Thesystem of claim 1, wherein the SLMs are transmissive SLMs.
 7. The systemof claim 1, wherein the SLMs are liquid crystal displays.
 8. The systemof claim 1, further comprising: a beam splitter that directs light fromthe light source to the SLMs and from the SLMs to the object, whereinthe SLMs are digital micromirror devices (DMDs).
 9. The system of claim1, wherein the control signal to the SLMs allows for more than one atleast partial exposure of a same area of the object during a singlescan.
 10. The system of claim 1, wherein the SLMs are positioned apredetermined distance apart such that more than one SLM exposes anexposure area on the object for pulses occurring during continuousmovement of the object based on the control signal.
 11. The system ofclaim 1, wherein the SLMs are configured in a two-dimensional array. 12.The system of claim 1, wherein a first edge of a first one of the SLMslies in a same plane as a second edge of an adjacent, second one of theSLMs.
 13. The system of claim 1, wherein the SLMs are staggered withrespect to adjacent SLMs.
 14. The system of claim 1, wherein a set ofthe SLMs that writes to a same exposure area on the object includes fourcolumns of the SLMs.
 15. The system of claim 14, wherein the columns arepositioned one-half of an active area apart.
 16. The system of claim 14,wherein each of the SLMs has an inactive area and wherein a top of theinactive area of one of the SLMs is aligned with a bottom of theinactive area of an adjacent one of the SLMS.
 17. The system of claim14, wherein each of the columns includes two of the SLMs.
 18. The systemof claim 14, wherein each of the columns includes four of the SLMs. 19.The system of claim 14, wherein the exposure area on the object moves adistance approximately equal to two active areas of the SLMs during eachpulse of the illumination system.
 20. The system of claim 1, wherein aset of the SLMs that writes to a same exposure area on the objectincludes six columns of the SLMs.
 21. The system of claim 20, wherein atop of an inactive area of one of the SLMs is aligned with a bottom ofan adjacent inactive area of another one of the SLMs.
 22. The system ofclaim 20, wherein each of the columns includes four of the SLMs.
 23. Thesystem of claim 20, wherein the columns are positioned one-half of anactive area apart.
 24. The system of claim 20, wherein the columns arepositioned one active area apart.
 25. The system of claim 1, wherein aset of the SLMs that writes to a same exposure area on the objectincludes two columns of the SLMs.
 26. The system of claim 25, wherein atop of an inactive area of one of the SLMs is aligned with a bottom ofan adjacent inactive area of another of the SLMs.
 27. The system ofclaim 25, wherein each of the columns includes four of the SLMs.
 28. Thesystem of claim 25, wherein the columns are positioned one-half of anactive area apart.
 29. The system of claim 25, wherein the exposure areaon the object moves a distance approximately equal to one active area ofthe SLMs during each pulse of the illumination system.
 30. The system ofclaim 1, wherein the SLMs are coupled to a support device in a twodimensional array.
 31. The system of claim 30, wherein the supportdevice comprises cooling channels running therethrough.
 32. The systemof claim 30, wherein the support device comprises control circuitry. 33.The system of claim 1, wherein each one of the SLMs comprise: a activearea section; and a package section.
 34. The system of claim 33, whereinthe package section comprises: control circuitry to control devices inthe active area.
 35. The system of claim 34, wherein the controlcircuitry receives the control signal from the controller.
 36. Thesystem of claim 1, wherein a set of the SLMs that writes to a sameexposure area on the object includes six columns of the SLMs.
 37. Thesystem of claim 1, wherein a set of the SLMs that writes to a sameexposure area on the object includes eight columns of the SLMs.